This invention relates to a device for estimating a size of an original document placed on a contact glass plate of an image forming apparatus such as a copying machine, and more particularly to an original document size estimating device which estimates a size of an original document by detecting a rear end edge of the original document.
In conventional size estimating devices for automatically estimating a size of an original document placed on a contact glass plate, numerous photosensors are provided at various positions under the contact glass plate so that the size of each original document can be logically estimated based on detection signals of these photosensors. There have been known size estimating devices in which a rear end edge of an original document is detected by a photosensor shiftable together with an optical system, and a size of the original document is estimated based on a detected position of rear end edge and a predetermined sheet size classification, e.g., A-standardized size or B-standardized size.
However, in the former devices, there has been the difficulty of placing numerous photosensors at appropriate places to logically and effectively detect an original document without causing interference with the optical system due to the fact that the optical system is normally located under the contact glass plate and reciprocates along the lower surface of the contact glass plate to read or scan images on the original document. Thus, a special arrangement has been required to avoid the interference between the numerous photosensors and the optical system, resulting in a complicated arrangement and bringing an undesirable increase of size.
Also, the latter devices have not been sufficiently reliable for the likelihood that an original document containing a relatively large black area is wrongly estimated to be a smaller size than an actual size because a border between the relatively large black area and a usual white background region is likely to be erroneously judged as a rear end edge. It will be seen that this wrong estimation results in a failure of copying an original image on an insufficient size copy sheet.